The present invention relates to a process for post-treating roughened and anodically oxidized aluminum, particularly support materials for offset printing plates, with aqueous solutions containing phosphoroxo anions.
Support materials for offset printing plates are provided, on one or both sides, with a radiation-sensitive (photosensitive) layer (reproduction layer), either directly by users or by manufacturers of precoated printing plates. This layer permits the production of a printing image of an original by photomechanical means. When a printing form is produced from the printing plate comprising such a reproduction layer, the layer support carries image areas which accept ink in the subsequent printing process. Concurrently, a hydrophilic image background for the lithographic printing operation is formed in the areas which are free from an image (non-image areas).
For the above reasons, the following requirements are demanded of a layer support for reproduction layers used in the manufacture of offset printing plates:
Those portions of the radiation-sensitive layer which have become comparatively more soluble following exposure must be capable of being easily removed from the support by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue.
The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water rapidly and permanently during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The radiation-sensitive layer must exhibit an adequate degree of adhesion prior to exposure, and those portions of the layer which print must exhibit adequate adhesion following exposure.
The base material employed for layer supports of this type preferably comprises aluminum. The base material is superficially roughened using known methods, such as dry brushing, wet brushing, sandblasting, chemical and/or electrochemical treatment. The roughened substrate then is optionally subjected to an anodizing treatment, during which a thin oxide layer is built up, to improve abrasion resistance.
In practice, the support materials, particularly anodically oxidized support materials based on aluminum, are often subjected to a further treatment step before applying a radiation-sensitive layer. This treatment improves the adhesion of the layer, increases the hydrophilic properties of the support and/or improves the developability of the radiation-sensitive layer. Such treatments are, for example, carried out according to the following methods:
German Patent No. 16 71 614 (corresponding to U.S. Pat. No. 3,511,661) essentially describes an anodic oxidation of support materials for printing plates in an aqueous solution of H.sub.3 PO.sub.4. In a comparative example (Example 12), a two-stage process variant is performed in which the support material is first anodically oxidized in an aqueous solution of H.sub.2 SO.sub.4 and then is post-treated by immersion into an aqueous solution of H.sub.3 PO.sub.4 or Na.sub.2 HPO.sub.4. The patent document teaches that it is necessary to apply a layer of hydroxyethyl cellulose before applying the layer comprising a radiation-sensitive composition.
German Offenlegungsschrift No. 22 51 710 (corresponding to British Patent Specification No. 1,410,768) also discloses the non-electrolytic post-treatment, in an aqueous solution of H.sub.3 PO.sub.4, of an aluminum support material for printing plates, the support material having been anodically oxidized in an aqueous solution of H.sub.2 SO.sub.4. A similar process is described by U.S. Pat. No. 3,808,000.
In the process for producing a support material for printing plates according to German Patent No. 25 40 561 (corresponding to U.S. Pat. No. 4,116,695), immersion in an aqueous solution containing an acid (for example, meta-, pyro- or polyphosphoric acid) or a base (or example, Na.sub.3 PO.sub.4 or K.sub.3 PO.sub.4) is performed as an intermediate stage, prior to a treatment with steam or hot water and following a customary anodic oxidation of the aluminum (for example, in an aqueous solution of H.sub.2 SO.sub.4).
In the two-stage process for the anodic oxidation of aluminum support materials for printing plates according to European Patent Application No. 0,086,957 (corresponding to South African Patent No. 83/0947), electrochemical treatment is performed (a) in an aqueous solution of H.sub.2 SO.sub.4 and (b) in an aqueous solution comprising phosphorus-containing anions (phosphoroxo anions, phosphorofluro anions and/or phosphoroxofluoro anions). The following are mentioned as suitable compounds for step (b):
phosphoric acid (H.sub.3 PO.sub.4): PA1 sodium dihydrogen phosphate (NaH.sub.2 PO.sub.4); PA1 disodium hydrogen phosphate (Na.sub.2 HPO.sub.4); PA1 trisodium phosphate (Na.sub.3 PO.sub.4); PA1 phosphorous acid (H.sub.3 PO.sub.3); PA1 disodium phosphite (Na.sub.2 HPO.sub.3); PA1 diphosphoric acid (H.sub.4 P.sub.2 O.sub.7); PA1 sodium pyrophosphate (Na.sub.4 P.sub.2 O.sub.7); PA1 triphosphoric acid (H.sub.5 P.sub.3 O.sub.10); PA1 sodium triphosphate (Na.sub.5 P.sub.3 O.sub.10); PA1 polyphosphoric acid (H.sub.n+2 P.sub.n O.sub.3 n+1); PA1 hexasodium tetrapolyphosphate [Na.sub.6 P.sub.4 O.sub.13 ]; PA1 hexasodium metaphosphate (Na.sub.6 (PO.sub.3).sub.6); PA1 disodium monofluorophosphate (Na.sub.2 PO.sub.3 F); and PA1 potassium hexafluorophosphate (KPF.sub.6).
It is true that these post-treatment processes often give satisfying results, but they cannot meet all of the increasingly stringent requirements made of a support material which is suitable for both the most up-to-date, practical applications and being coated with the most diverse radiation-sensitive reproduction layers. In particular, the known processes do not satisfy the requirements for an uncomplicated and inexpensive method for producing such a support material. This drawback to the known processes applies not only to the resistance to alkaline media, which is of particular importance when high-performance developers are used with positive-working, radiation-sensitive reproduction layers, but also to the adsorption characteristics of the oxide layers. The adsorption values are important, since staining (e.g., coloration) of the non-image areas, which most probably is caused by adsorptive effects, can occur, depending on the chemical composition of the reproduction layers.
For the industrial manufacture of these support materials in high-speed, high-performance installations, it is also desirable to develop a process for the post-treatment of oxide layers which can be performed in an energy-efficient manner, with the lowest possible malfunction rate. This means, for example, that an immersion treatment is generally preferred over an electrochemical treatment, if the respectively treated oxide layers exhibit comparable surface properties.